Early enough that the revenue story rests on signed work rather than forecasts. A pipeline built during the months before filing turns marginality from an argument into evidence. Set the milestone by what has been contracted, not by a preferred launch month, and accept that preparation, any consular step, government review and admission each take their own time.
Use evidence milestones, not calendar promises
Set checkpoints that describe a state of the file: costs priced and committed, company registered and banking, three client agreements signed, plan and financials reviewed. Move to the next stage when the checkpoint is met rather than when the date arrives. Nobody can promise an approval or a decision date, and estimates change.
Keep any resignation, lease or school enrolment decision behind the checkpoint that would actually make it safe. Add one further checkpoint before the others: confirm that nothing in the plan requires the founder to be working in the business before the status permitting it exists. Preparing to open — registering the company, opening accounts, negotiating with suppliers, signing client agreements as a prospective owner — is different from operating it, and a service business whose first client engagement falls before that line has a problem that no amount of subsequent documentation resolves.
Decide who may lawfully act during the interval. Hypothetical example: a founder opening an alterations and tailoring service has three clients wanting work in a month that falls before the position is resolved, and arranges for a hired seamstress to perform the work under a written arrangement rather than moving the dates twice.